The present invention relates to a method for forming a pattern, and more particularly, to a method for forming a pattern having a simple manufacturing process while providing an improved profile at the portion of a high step.
Increased integration and performance demands have caused complicated structures to be introduced in semiconductor devices. Accordingly, a method for forming a fine pattern on a semiconductor substrate is required. It is widely known that patterns may be formed by a photolithographic method.
The photolithographic pattern-forming method is as follows. First, a photoresist layer whose solubility may be changed by irradiation with light, such as ultraviolet or X-ray, is formed on the upper surface of a substrate. For example, the upper surface may be an insulation layer, a conductive layer or a semiconductor wafer on which a pattern may be formed. Light is selectively irradiated onto the resist layer using a mask which exposes the resist in a predetermined pattern. Thereafter, a resist portion having a high solubility is removed, while a resist portion having low solubility is left to form a resist pattern. In the case of a positive resist, a developing step removes the exposed portion. The portion of the substrate from which the resist is removed is etched or otherwise processed to form a pattern. After completing substrate processing, the remaining resist is removed.
Although a pattern can be formed, the resolution of photolithography is limited. That is, after exposing the photoresist layer to light and developing the exposed photoresist, the line width of the formed pattern is required to have the same resolution as the line width of a photomask pattern. However, it is very difficult to maintain a constant linewidth of the pattern. Such linewidth deviations are due to energy dosage differences in resists having different thicknesses, and due to light scattering resulting from refraction and reflection (see "Silicon Processing for the VSLI Era" by S. Wolf and R.N. Tauber, Vol. 1, p.439, 1986).
FIG. 1 is a flowchart diagram of a pattern formation process according to the conventional single-layer resist method. The photosensitive material is spin-coated on the upper surface of the substrate while the substrate rotates at a velocity of 100 to 1,000 rpm. Thereafter, if the substrate is rotated at a higher speed (e.g., 2,000 to 6,000 rpm), centrifugal force spreads the photosensitive material and forms as a resist layer over the whole substrate.
FIGS. 3A and 3B illustrate shortcomings of spin coating a resist. FIG. 3A shows a cross sectional view of a substrate 1 coated with a resist material 2 in a region where the substrate has a step change in height. Although the resist thickness may be fairly uniform at locations away from the step, the resist thickness varies in a region A in the vicinity of the step.
FIG. 3B shows a surface view of a substrate with resist after developing. The resist has been patterned and developed to form lines which cross the step region. In locations away from the step, the resist development process leaves lines of resist 3a and lines of exposed substrate 3b. However, in the vicinity of the step, in lines which should have exposed substrate, some resist material 4 remains.
This undesirable result can occur because the resist is thicker in the vicinity of the step, and the resist material is not uniformly exposed. Typically, optical equipment used to expose the resist material focuses the pattern at a focal plane. The focal plane is adjusted to lie between the surface of the resist material and the substrate. However changes in substrate height makes it difficult to align the focal plane precisely with the center of the resist layer. Because of the increased resist thickness in the vicinity of the step and alignment of the focal plane, material close to the substrate may not be fully exposed. For example, the pattern may be out-of-focus at the bottom of the step, and diffusion of the light energy over a greater amount of material may result in insufficient exposure. The result may be undesirable variations in line thickness, bridging, scum, etc.
To solve the above problem of the conventional single-layer resist (SLR) method described above, a multi-layer resist (MLR) method has been developed.
FIG. 2 shows a flowchart according to the MLR method and FIG. 4 shows an interlayer structure during pattern formation according to the MLR method.
First, on the upper portion of substrate 1, a lower photoresist 11 is applied to form a planarized surface and then baked. After an insulation layer 12 is coated on the upper portion of the lower photoresist 11, an upper photoresist 13 is coated to form a multi-layer resist. After the upper photoresist is exposed and developed, the insulation layer 12 is dry-etched and the lower photoresist 11 is etched using oxygen gas (O.sub.2) plasma to form a resist pattern. Then, using the resist pattern, the substrate is etched (or otherwise processed), and the remaining resist is removed. The MLR method can yield patterns having resolution less than or equal to 0.5 .mu.m.
One application of the MLR method is disclosed in Japanese Laid-open Patent Publication sho 51-107775. In the above Japanese publication, an organic layer (lower layer) is thickly coated onto a substrate in which uneven portions or steps exist so as to planarize the upper portion of the substrate. An intermediate layer composed of a spin on glass (SOG), a phosphosilicate glass (PSG) or SiO.sub.2 is formed, and then a photoresist layer (upper layer) is formed on the intermediate layer. The upper layer is exposed and developed according to the conventional photolithographic techniques, and the intermediate layer is etched. Then, using the intermediate layer as a mask, the portion of the thick layer (exposed through the intermediate layer) is dry-etched. The exposed portion of the substrate is dry-etched to form a predetermined pattern.
Also, Korean Patent Publication No. 89-3903 discloses a method for improving a resolution of a pattern in the MLR method. By reducing differences between the refractive indices of the intermediate layer with respect to the upper and lower layers, light reflection is reduced at the layer boundaries. At the same time, interference which occurs due to the difference in layer thickness is prevented, which in turn reduces pattern variations.
The method described in the above-mentioned Korean patent publication comprises the steps of: 1) sequentially depositing a lower layer of organic polymer, an intermediate layer made of a material having a larger resistance to dry-etching than the lower layer and a photosensitive upper layer; 2) exposing the upper layer with a pattern and developing the exposed portion; 3) removing an exposed portion of the lower layer; 4) and forming a substrate pattern using the lower layer as a mask. The differences between the refractive indices of the intermediate layer relative to the upper and lower layers are less than or equal to 12%.
The fine pattern having a high resolution can be obtained by the MLR method. However, since the processes are complicated productivity is low, the number of defective products increases and the entire cost of the semiconductor manufacturing process increases.